The main task of the present paper is to investigate the nature of collective knowledge and discuss what kind of justificatory aspects are involved in it to discuss it from collective belief. The central kind of collective knowledge investigated is normatively binding knowledge attributed to a social group. A distinction is made between natural knowledge and constitutive knowledge related to social (especially institutional) matters. In the case of the latter kind of knowledge, in contrast to the former kind, justification and the criteria of justification are purely social. Knowledge is regarded as a primitive, irreducible notion that accordingly does not fall prey to Gettier-type paradoxes.